1. Field of the Invention
The invention relates to a motor drive circuit.
2. Description of the Related Art
A fan motor is used in a personal computer or home electronics in order to cool a heat generating component. When a heat generating component is cooled by the fan motor, the cooling performance is enhanced by increasing the rotation speed of the motor, but it is necessary to adjust the rotation speed of the motor appropriately corresponding to the amount of heat generation in order to minimize the power consumption or reduce the noise.
There is a PAM (Pulse Amplitude Modulation) control method as a type of motor rotation control method. In the PAM control method, the rotation speed of a motor is adjusted by increasing or decreasing a drive voltage applied to the motor coil corresponding to the amount of heat generation.
Furthermore, in order to enhance the minimization of the power consumption or the noise reduction, intermittent drive may be employed as well as the drive voltage control. For example, Japanese Patent Application Publication. No. 2006-174648 discloses a control method in which a drive voltage is decreased corresponding to the target rotation speed of a motor and a ratio of a drive voltage applied to the motor coil is decreased as the target rotation speed of the motor is decreased. This control method realizes a control such that a motor rotates at lower speed compared with a case of using the drive voltage adjustment only, thereby realizing the minimization of the power consumption or the noise reduction.
During the rotation of such a motor, a cogging torque occurs by attraction or repulsion corresponding to the positional relation of the magnetic poles and the motor coil. During the stop of the motor, the motor coil often rests on a position where the cogging torque becomes minimum. Therefore, in order to start the rotation of the motor that is stopping, a torque exceeding the maximum level of the cogging torque is necessary.
In the method disclosed in Japanese Patent Application Publication No. 2006-174648, when the rotation speed of the motor is decreased, the drive voltage is low and the driving ratio is also decreased, and thus the torque for driving the motor is also decreased. During the rotation of the motor, the rotation is kept by inertia even with a small torque. However, in order to start the rotation of the motor at low speed when the motor is stopping, the torque for driving the motor can not exceed the maximum level of the cogging torque, resulting in a failure of start at low speed. In particular, a single-phase motor has a larger difference between the maximum level and the minimum level of the cogging torque, compared with a three-phase motor, starting at low speed is more difficult.